docs: def infer_x(self, y):
          """Infer probable x from input y
  
          @param y  the desired output for infered x.
          """
          assert len(y) == self.fmodel.dim_y, "Wrong dimension for y. Expected %i, got %i" % (self.fmodel.dim_y, len(y))
          if len(self.fmodel.dataset) == 0:
              return [[0.0]*self.dim_x]
          else:
              _, index = self.fmodel.dataset.nn_y(y, k=1)
              return [self.fmodel.dataset.get_x(index[0])]